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The present study describes generally the ecosystem of Lake Verevi while more detailed approaches are presented in the same issue. The main task of the article is to estimate long-term changes and find the best method for the restoration of good ecological status. Lake Verevi (surface 12.6 ha, mean depth 3.6 m, maximum depth 11 m, drainage area 1.1 km, water exchange 0.63-times per year) is a hypertrophic hardwater lake located in town Elva (6400 inhabitants). Long-term complex limnological investigations have taken place since 1929. The lake has been contaminated by irregular discharge of urban wastewaters from oxidation ponds since 1978, flood from streets, and infiltrated waters from the surrounding farms. The socalled spring meromixis occurred due to extremely warm springs in recent years. The index value of buffer capacity of Lake Verevi calculated from natural conditions is on the medium level. Water properties were analysed according to the requirements of the EU Water Framework Directive. According to the classi- fication, water quality as a long-term average of surface layers is moderate-good, but the water quality of bottom layers is bad. Values in deeper layers usually exceed 20–30 times the calculated reference values by Vighi and Chiaudani’s model. Naturally, at the beginning of the 20th century the limnological type of the lake was moderately eutrophic. During the 1980s and 1990s the ecosystem was out of balance by abiotic characteristics as well as by plankton indicators. Rapid fluctuations of species composition and abundance can be found in recent years. Seasonal variations are considerable and species composition differs remarkably also in the water column. The dominating macrophyte species vary from year to year. Since the annual amount of precipitation from the atmosphere onto the lake surface is several times higher, the impact of swimmers could be considered irrelevant. Some restoration methods were discussed. The first step, stopping external pollution, was completed by damming the inlet. Drainage (siphoning) of the hypolimnetic water is discussed. Secondary pollution occurs because Fe:P values are below the threshold. The authors propose to use phosphorus precipitation and hypolimnetic aeration instead of siphoning.

Mass balances of total nitrogen and total phosphorus were calculated for Lake Verevi (area 0.126 km, maximum depth 11 m, mean depth 3.6 m), a sharply stratified small lake located in South Estonia within the borders of the town Elva. The lake has up to 10 small inflows but only three of them are nearly permanent. Accidental overflows from near-by oxidation ponds during high floods have been the major source of the nutrient load of the lake in the past. L. Verevi receives a significant part of its inflow from groundwater, which is difficult to measure. In dry years the outflow is temporary. During summer the lake is sharply thermally and chemically stratified. The spring turnover is often incomplete even in homothermal conditions, thus giving the lake some meromictic features. The influx of nitrogen exceeded the outflux at any supposed proportion (20%, 50%, 80%) of surface runoff. The lake retained 45–90% of the nitrogen influx by sedimentation and/or by denitrification. The largest nitrogen losses with loss rates more than 10 kg N d occurred in May and June. The calculated phosphorus retention rate became strongly negative during mixing periods. From June to November, phosphorus release from the sediment exceeded sedimentation by 205 kg in 1991 and by 79 kg in 1993. Earlier stagnation and absence of a full spring turnover in the 2000 has slowed down the recovery of the lake because less phosphorus is flushed out. However, the stronger stratification and significantly smaller phosphorus content in the epilimnion limits biological activity and as a result improves the water quality of the surface layer.

Lake Verevi is a hypertrophic and strongly stratified (partly meromictic) small temperate lake. Vertical distribution of sediment phosphorus fractions as well as iron, manganese, organic matter and calcium carbonate of the deep bottom sediment was determined. The study focused on the ecologically important layer of the sediment [<20(45)cm]. In the uppermost layers of the sediment, NaOH-NRP (organic P) dominated while HCl-RP (apatite-P) became dominant in some deeper layers below 7 cm. Extremely high concentrations of labile phosphorus fraction (NHCl-RP) indicated the low binding capacity of phosphorus by lake sediment. Due to sediment and hypolimnion anoxia, the internal load of phosphorus in this lake is most likely. Potentially mobile phosphorus fractions (NHCl-RP, BD-RP, NaOH-NRP) formed 301 kg in upper 10 cm thick sediment layer of hypolimnetic bottom sediment (40% of lake bottom area).

The optical properties and light climate during the ice-free period in the highly stratified Lake Verevi (Estonia) have been studied together with other lakes in same region since 1994. The upper water layer above the thermocline belongs to class “moderate” by optical classification of Estonian lakes but can turn “turbid” (concentration of chlorophyll up to 73 mg m and total suspended matter up to 13.2 g m) during late summer blooms. In the blue part of the spectrum, light is mainly attenuated by dissolved organic matter and in red part notably scattering but also absorption by phytoplanktonic pigments effect the spectral distribution of underwater light. Consequently, the underwater light is of greenish-yellow color (550–650 nm). Rapid change in optical properties occurs with an increase of all optically active substances close to thermocline (2.5–6 m). Optical measurements are often hampered beneath this layer so that modeling of the depth distribution of the diffuse attenuation coefficient is an useful compliment to field measurements. K ranges from 0.8 to 2.9 m in the surface layer, and model results suggest that it may be up to 5.8 m in the optically dense layer. This forms a barrier for light penetration into the hypolimnion.

The small strongly stratified hard-water hypertrophic lake Verevi (max. depth 11.0 m, surface area 12.6 ha, mean depth 3.6 m) was investigated in 2000 and in 2001. The lake is sheltered from winds, and the role of waves in mixing the water column is minimal. Eutrophication favours the strengthening of stratification. Early warm springs cause a fast stagnation of the water column forming partly meromictic conditions. Seston content of water and in sediment traps in 3 layers was measured several times during the formation of stratification. Besides measuring particulate matter, in 2001, the nutrient content of the trapped sediment was analysed. During the first 7 days of the investigation, 30% of the total particle sedimentation took place. The sedimentation rate of particulate matter was 0.4–6.3 g m d dry weight in different layers of the water column. Daily average sedimentation loss rate was 27% of the total amount of seston of the epilimnion, whilst from the meta- and hypolimnion the settling was much slower (9.6 and 7.3%, respectively). In our experiments with twin sediment traps, to one of which formaldehyde was added, the PO{sk4/3−}-P concentration was 19% smaller in the trap without formaldehyde, probably due to planktonic uptake. The relationship between primary and export production is loop-like. The shape was irregular, indicating a high grazing rate of zooplankton.

The dynamics of different nitrogen compounds and nitrification in diverse habitats of a stratified Lake Verevi (Estonia) was investigated in 2000–2001. Also planktonic N-fixation (Nfix) was measured in August of the observed years. The nitrogen that accumulated in the hypolimnion was trapped in the nonmixed layer during most of the vegetation period causing a concentration of an order of magnitude higher than in the epilimnion. The ammonium level remained low in the epilimnion (maximum 577 mgN m, average 115 mgN m) in spite of high concentrations in the hypolimnion (maximum 12223 mgN m, average 4807 mgN m). The concentrations of NO{sk2/−} and NO{sk3/−} remained on a low level both in the epilimnion (average 0.94 and 9.09 mgN m, respectively) and hypolimnion (average 0.47 and 5.05 mgN m, respectively). Nfix and nitrification ranged from 0.30 to 2.80 mgN m3 day 1 and 6.0 to 107 mgN m3 day 1, respectively; the most intensive processes occurred in 07.08.00 at depths of 2 and 5 m, accordingly. The role of Nfix in the total nitrogen budget of Lake Verevi (in August 2000 and 2001) was negligible while episodically in the nitrogen-depleted epilimnion the Nfix could substantially contribute to the pool of mineral nitrogen. Nitrification was unable to influence nitrogen dynamics in the epilimnion while some temporary coupling with ammonium dynamics in the hypolimnion was documented.

Vertical distribution of phytoplankton and the formation of deep chlorophyll maximum (DCM) in the metalimnion of a small stratified and partly meromictic temperate lake was studied in 1999 and 2000. During summer DCM usually occurred on the borderline of HS and oxygen-containing waters. At the depths where the bacteriochlorophyll (Bchl) maxima were observed, the sulphide concentration was usually relatively low compared to the bottom layers, where its concentration reached as high as possible saturation level. In April 2000, DCM was formed at the depth of 3.5 m, and lowered thereafter slowly to 6.5 m by October. The concentration of Bchl reached the highest values (over 1000 g l) just before the water column was mixed up in autumn. In December and April Bchl was detectable only near the bottom of the lake. The concentration of chlorophyll yielded by the spectrophotometric phaeopigment corrected method and by HPLC (high pressure liquid chromatography), fit rather well in the upper layers. In deeper water layers chlorophyll concentration (Chl ) measured by spectrophotometry was overestimated about 47 times if compared to HPLC values because of the high Bchl in that layer. In most cases vertical profiles of primary production (PP) did not coincide with the vertical distribution of the pigment content; the maximum values of PP were found in the epilimnion. In some cases PP had notably high values also at the depth of DCM. In the upper layers Chl usually did not exceeded 20 g l1 in spring and 10 g l1 in summer. The moderately high Chl in the epilimnion in spring was significantly reduced after the formation of thermocline most probably because of the establishment of the nutrient limitation in epilimnion. Decreasing Chl concentration in the epilimnion led to increased water transparency and better light conditions for photosynthetic bacteria in metalimnion.

Bacterioplankton abundance and production were followed during one decade (1991–2001) in the hypertrophic and steeply stratified small Lake Verevi (Estonia). The lake is generally dimictic. However, a partly meromictic status could be formed in specific meteorological conditions as occurred in springs of 2000 and 2001. The abundance of bacteria in Lake Verevi is highly variable (0.70 to 22 × 10 cells ml) and generally the highest in anoxic hypolimnetic water. In 2000–2001, the bacterial abundance in the hypolimnion increased probably due to meromixis. During a productive season, heterotrophic bacteria were able to consume about 10–40% of primary production in the epilimnion. Our study showed that bacterioplankton in the epilimnion was top-down controlled by predators, while in metalimnion bacteria were dependent on energy and carbon sources (bottom-up regulated). Below the thermocline hypolimnetic bacteria mineralized organic matter what led to the depletion of oxygen and created anoxic hypolimnion where rich mineral nutrient and sulphide concentrations coexisted with high bacterial numbers.

Changes in the phytoplankton community of the hypertrophic, sharply stratified Lake Verevi have been studied over eight decades. Due to irregular discharge of urban wastewater, the trophic state of the lake has changed from moderately eutrophic to hypertrophic. We found that the trophic state in summer increased in the 1980s and remained at a hypertrophic level since then. was recorded first in the 1950s and became the dominant species in the 1980s, forming biomass maxima under the ice and in the metalimnion during the vegetation period. In summer 1989, contributed almost 100% of the phytoplankton biomass. Generally, the highest biomass values occurred in the metalimnion. In spring, when was less numerous, diatoms and cryptophytes prevailed. In springs 2000 and 2001 different diatoms dominated — var. (18.6 g m) and (9.2 g m), respectively. In recent years, the spring overturn has been absent. In the conditions of strong thermal stratification sharp vertical gradients of light and nutrients caused a large number of vertically narrow niches in the water column. During a typical summer stage, the epilimnion, dominated by small flagellated chrysophytes, is nearly mesotrophic, and water transparency may reach 4 m. The lower part of the water column is hypertrophic with different species of cryptophytes and euglenophytes. A characteristic feature is the higher diversity of Chlorococcales. Often, species could form their peaks of biomass in very narrow layers, e.g. in August 2001 (18.6 g m) was found at a depth of 5 m (the lower part of the metalimnion with hypoxic conditions), spp. (56 g m) at 6 m (with traces of oxygen and a relatively high content of dissolved organic matter) and euglenophytes (0.6 g m) at 7 m and deeper (without oxygen and a high content of dissolved organic matter).

Lake Verevi (12.6 ha, maximum depth 11.0 m, mean depth 3.6 m) is a strongly eutrophic and stratified lake. is the most characteristic phytoplankton species in summer and autumn, while photosynthesizing sulphur bacteria can occur massively in the metalimnion. Primary production (PP) and chlorophyll a concentration (Chl ) were seasonally studied in 1991, 1993, 2000, and 2001. Vertical distribution of PP was rather complex, having usually two peaks, one at or near the surface (0–1 m), and another deeper (at 3–7 m) in the metalimnion. The values of dark fixation of CO in the metalimnion were in most cases higher than those in the upper water layer. Considering the average daily PP 896 mg C m and yearly PP 162 mg C m, Secchi depth 2.34 m, and epilimnetic concentrations of chlorophyll a (19.6 mg m), total nitrogen and total phosphorus (TP, 52 mg m) in 2000, L. Verevi is a eutrophic lake of a ‘good’ status. Considering the total amounts of nutrients stored in the hypolimnion, the average potential concentrations in the whole water column could achieve 1885 mg m of TN and 170 mg m of TP reflecting hypertrophic conditions and a ‘bad’ status. Improvement of the epilimnetic water quality from the 1990s to the 2000s may have resulted from incomplete spring mixing and might not reflect the real improvement. A decreased nutrient concentration in the epilimnion has supported the establishment of a ‘clear epilimnion state’ allowing light to penetrate into the nutrient-rich metalimnion and sustaining a high production of cyanobacteria and phototrophic sulphur bacteria.